A. Field of the Invention
This invention pertains to a system and method for inserting copy control information into an analog signal, including a right assertion mark and a CGSM signal.
B. Description of the Prior Art
Copyrighted works normally contain associated rights information. In the case of a book, for example, this information might consist of the name of the author(s) and the date from which copyright is claimed. In more complex cases, such as those related to transmission of audio-visual works over digital transmission media, it may be desirable to convey rights information that goes beyond such basic information. For example, rights information with respect to audio-visual works could, include information on whether copies may be made of the works, how many copies may be made, and whether the works may be retransmitted to another party.
There are various methods of transmitting audio-visual works from a source (the point at which the transmission originates, such as a set-top box, or an optical disc player, such as a DVD player) to a sink (the point at which the work is displayed or recorded for later display). Such methods include robust digital connections using secure transmission technologies such as DTCP on IEEE-1394 high-speed digital links or HDCP on DVI digital links. The most common method for transferring works from a source to a sink utilizes an analog video connection with the transmission being an NTSC or PAL composite video signal. However, when using current techniques, analog delivery does not provide robust protection for a work.
In the context of the protection of audio-visual works, the word “robust” refers to the strength of the copy protection. In analog signaling, the term is used to mean that rights information is transmitted in a way that makes it unlikely that the information would be completely removed inadvertently. In digital signaling, the term indicates that a secure mechanism is used that involves the authentication of the sink device by the source device. Authentication ensures that the source device can trust the sink device to follow the rules. Such secure mechanisms can include the encryption of the rights information and the work during transmission so that any attempt to monitor the transmission will fail. In addition, the rights information can be accompanied by a digital signature, an unforgeable attachment to the rights information that will indicate if unauthorized alterations are made to rights information.
In general, digital links can be robust because the signaling is two-way, while analog links are limited in their robustness in that the sink device cannot typically communicate with the source device.
Unfortunately, in converting a digital work to analog form (for example, an NTSC video signal), the rights information associated with the digital form of the work may be lost or translated into a form that can be subsequently removed. The conversion of the work to an analog video signal is necessary to allow the work to be viewed on a conventional television set or video monitor, the majority of which have only analog video inputs. However, the conversion of the digital form of the work to analog followed by the re-conversion back to a digital form can be used as a means of circumventing the content protection of the work in a digital form.
The problem of the loss of rights information this way is known as the “analog hole” and there are efforts underway to devise ways to “plug” the hole. While certain technologies have been identified and used in an attempt to prevent the loss of rights information upon conversion of digital audio-visual works to analog, there is no equivalently robust protection for protecting analog transmissions of such works that has been accepted for widespread use.
One technology that has been used for the transmission of rights information in an analog video signal is called CGMS-A. CGMS-A is a standard that conveys copy control information (CCI) in the non-visible vertical blanking interval (VBI) of a video signal (FIG. 1). The VBI is the period during which the display device resets its scan line from the bottom of the picture to the top. In NTSC encoded video signals, the CGMS-A information is carried on either or both of lines 20 and 21 of the picture (FIG. 2), which lines are transmitted during the VBI. CGMS-A carries a 2-bit CCI value which can convey the information “copy never”, “copy one generation”, “copy control not asserted” and, in most definitions of CGMS-A, a fourth state of “copy no more.”
Redistribution of an audio-visual work occurs when that work is made available other than through the original distribution means authorized by the rights owner. For example, an audio-visual work is “redistributed” if it is recorded from a free-to-air broadcast and the recording is then made available on the Internet. In-home recording of free-to-air broadcasts is popularly understood to be permitted for the private use of the person making the recording. It is not permitted to take a recording made in this way and make it available to others. To do so constitutes redistribution. Recently a retransmission control (RC) bit has been defined in some CGMS-A standards such as 805A. The RC flag is designed to signal that the content cannot be retransmitted. The RC bit is a third bit of CCI. Herein, the term CGMS-A will include the RC bit unless specifically stated otherwise.
Unfortunately the CGMS-A information occurs in a part of the signal that is not displayed and consequently the loss or removal of the information carried in this part of the signal does not affect the quality of the picture (although it may cause the loss of other information carried in this part of the picture such as closed caption text). CGMS-A is, therefore, not a robust signaling method. Not only can the CGMS-A bits be removed, but because CGMS-A is not required to be present in the video signal and is also trivial to remove, it is not possible to tell whether a video signal has had CGMS-A removed or whether it was even there to begin with.
Historically, watermarks are designs pressed into paper at the time of manufacture to identify the maker. A paper watermark is almost completely invisible until the paper is held up to the light. Watermarking is also the term used for the process of embedding a concealed mark that conveys or indicates rights ownership in a still picture, or video or audio signal such that it is not apparent to the viewer/listener.
Detection of CGMS-A information is simple, requiring no more complexity than is required to interpret an analog video signal for digitization or display. By comparison, the detection of embedded watermarks is often complex, involving advanced signal processing particularly when the watermark to be detected is embedded in the image. The added complexity of detecting a watermark normally adds cost to the device that is required to detect the watermark. Nevertheless, it has been proposed to combine CGMS-A and a watermark in an audio-visual work to provide greater robustness to the analog form of the work.
U.S. Pat. No. 6,433,946 in the name of Ogino that issued on Aug. 13, 2002 and is entitled “Information Signal Output Control Method, Information Signal Duplication Prevention Method, Information Signal Duplication Prevention Device, And Information Signal Recording Medium” proposes using a combined watermark and CGMS-A approach to control play and copying (but not redistribution) of an audio-visual work. The CGMS-A mark is not used in the usual way. Instead, it represents a “don't copy” command only. Not only does the system utilize less information than can be represented by a CGMS-A code, but the watermark in the first embodiment of the Ogino system is redundant with the CGMS-A code. If both signals are present, it means that the audio-visual work has not been tampered with, i.e., it is not an illegal copy. Copies are not permitted in accordance with the two codes, but the work is played. But if only one of the two codes is present, it is an indication that there has been tampering. In this case, not only is copying disabled, but the work is not even played. Only if both signals are absent is copying allowed along with play. In this embodiment of the Ogino system, there is symmetry between the watermark and the CGMS-A signals—the same copy and play controls apply if only one of the signals is present, regardless of which one it is.
The second Ogino embodiment is the same as the first, with one difference. If the GGMS-A signal is present but the watermark is not, play is not inhibited. But it is accompanied by a message that the play is improper. Play is allowed so that the user will not think that there is anything wrong with his television set, but the play is accompanied by interference. The basic control mechanism is the same as the first—neither signal takes precedence over the other, and the presence of one signal without the other is an indication that something improper has taken place before the audio-visual work reached the television set. (In a third embodiment of the Ogino system, which is more sophisticated in that “re-marking” to a “copy no more” state apparently takes place when a work with a “copy one generation” code is played, it is assumed that the CGMS-A signal is already on an audio-visual work, and a watermark is simply added based on the CGMS-A signal.)
A major shortcoming of the Ogino system is that in treating the CCI code and the watermark symmetrically, the CCI code can be used to represent no more than the watermark. That is why the state tables in the Ogino patent drawing simply have “yes” and “no” entries for whether the two signals are present or not. Because the watermark only represents “yes” or “no”, and the CCI code is treated symmetrically, the CCI code is restricted to represent only “yes” or “no” as well. The full range of information that is possible with a CGMS-A code is just not used. Instead of extending use of CGMS-A to a third CCI bit, the retransmission control (RC) bit to indicate whether or not the content can be retransmitted, Ogino does not even use the full potential of CGMS-A in its original form.
In contrast to the limitations of Ogino, it is an object of our invention to provide CCI bits (e.g., in the CGMS-A format) in the non-visible vertical blanking interval of a video signal in such a way that, if the bits are removed, it will be possible to tell that they have been removed so that appropriate decisions about play and redistribution of the underlying audio-visual work may be made, the CCI bits conveying more information than simply “copy” and “don't copy.” This is accomplished by inserting a video subcarrier component into the visible portion of the video signal.